Transmission of Infection Flashcards
What is the chain of infection?
infectious agent > reservoir > portal of exit > means of transmission > portal of entry > susceptible host
What is virulence?
The ability of the microbe to cause disease
What is dose?
The number of microbes entering the body
What is the international unit for dose?
Infectious dose 50 (ID50)
Why are viruses grown on tissues?
they cannot grow in agar
What does a low id mean and what is ID?
low id = more infectious
id values compares infectivity of microbes across different species
ID50 is the amount of pathogen (dose) required to infect half of a sampled population
What two categories do virulence factors usually include?
exotoxins
endotoxins
What are examples of exotoxins?
P. gingivalis (protease)
S. aureus (enterotoxin & leukocidin)
What is an example of a endotoxin?
Lipopolysaccharide
(released by P. gingivalis & E. coli)
Where does e.coli usually infect?
urinary tract
Which reservoir do most pathogenic microbes that infect humans come from?
other humans
What are examples of different reservoirs for microbes?
- humans
- animals (anthrax)
- environmental (clostridium tetani spores in soil)
- fomites
What type of bacteria is clostridum tetani?
gram positive bacillus
When are microbes not easily recognised in patients?
when patients are health carriers (convalescent and asymptomatic)
What is the incubation period?
the time between contamination and the development of symptoms
What do long incubation periods mean?
- longer time periods when the infecting microbe may be spread to others
- greater spread of the disease because of more human contact
What is a asymptomatic carrier?
An infected person with no clinical evidence of disease, though signs and symptoms of the disease may have been evident earlier
What is a carrier?
(as well as their contacts) are usually not aware of their infectious state
What is colonisation?
the presence of micro-organism (s) in or on a host, with growth and multiplication, but without any overt clinical expression (infection) at the time the micro-organism is isolated.
What is an endogenous reservoir and why does it occur?
Caused by members of the normal flora if:
Flora becomes ecologically harmful due to population/gene expression shifts e.g. periodontal disease and dental caries
they become displaced to another body site or are allowed to invade deeper tissues (e.g. post-surgical infections)
Is influenza endogenous or exogenous?
exogenous
What did general dentists have raised antibodies to?
Flu A
Flu B
RSV
What is the portal of exit?
Microbes must ESCAPE from the source to colonise a new host
* Mechanisms of escape vary depending upon the source.
What can the mode of escape be?
- NATURAL (e.g. coughing or sneezing)
- ARTIFICIAL (e.g. blood donation or dental handpiece aerosols)
What does COSHH classify human pathogens based on?
ability to cause infection (higher virulence)
severity of disease
vaccine and treatment availability
risk of population spread
What is the Ro?
reproductive number
the number of cases one case generates on average
over the course of its infectious period.
Ro < 1 VS Ro > 1
When Ro< 1 infection will die out in the long run
When Ro>1 infection will be able to spread in a population
What factors affect the Ro?
Duration of infectivity
Infectiousness
Number of susceptible people
How do respiratory viruses exit?
Droplets & aerosols Respiratory secretions
Airborne vs Droplet
precautions
Airborne precautions = less than 5 microns, tiny particles, travel further and remain in air longer (respiratory precautions)
Droplet precautions = more than 5 microns, large particles (non respiratory precautions)
What respiratory infections are accepted to be mostly transmitted by the airborne (aerosol route)?
a. tuberculosis,
b. measles
c. chickenpox
If the particle is larger, will it fall slower or faster than smaller?
larger = slower
What is an aerosol?
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas
What does survival depend on?
persistence, stability, retention of infectivity
What is fomite mediated transmission dependant on?
Environmental factors
Time between reservoirs
Efficiency of transfer
Dose of virus
Why is fomite transmission difficult to prove?
respiratory transmission from asymptomatic people cannot be ruled out
What is a barrier to fomite transmission?
hand hygiene
How can aerosols be mitigated in dentistry?
-Use of rubber dam
-High/low volume aspiration
-Surgery ventilation
How many air changes per hour?
10
Where are the portals of entry?
*Respiratory tract (influenza)
*Gastro-intestinal tract (norovirus)
*Open/surgical wounds (staph. aureus)
*Medical devices/Sharps injuries (Hep b)
*Sexual contact (HIV)
How can influenza virus enter?
*Respiration (aerosols)
*Inhalation (all particles)
*Direct contact (droplets)
*In-direct contact (settled particles – face touching)
* Role of conjunctiva uncertain
What is a susceptible host?
Is a person who is at risk of infection as they are unable to fight the infection due to;
*An underdeveloped immune system (neonate)
*A declining immune system (elderly)
* Disease or drugs that impair host defences
*Breaks in the skin
*Medical devices
What are the risk groups?
Elderly, young children, pregancy & chronic medical conditions (diabetes, asthma, heart disease)
